1. Field of the Invention
The present invention relates to a method for manufacturing components on a multi-layered silicon wafer by bonding together two initial wafers.
2. Description of the Related Technology
Such a method is known from the publication U.S. Pat. No. 5,583,059. This describes the integration of a so-called xe2x80x9cfully depletedxe2x80x9d n- and p-FET in combination with an npn heterobipolar transistor (HBT) on a wafer with an insulating intermediate layer (SOI wafer) which has an active layer thickness of less than 0.2 xcexcm. The silicon layer around the active components is completely removed in order to isolate the individual components and reduce the parasitic capacitances. Furthermore, the thickness of the active layer in the area of the npn transistor is increased by means of an epitaxy process in order to form a collector layer for the manufacture of an npn transistor. Provided that no selective epitaxy process is performed, protective layers, which are removed again in subsequent process steps, are applied in order to protect the FET components. The base of the HBT is also deposited during the multi-stage epitaxy process, during which a so-called drift-HBT is created which does not have a low-doped emitter layer between emitter contact layer and base layer.
The disadvantage of the process architecture described is that the thickness of the active layer has to be substantially increased in the area of the npn transistors in order to adapt electrical parameters such as early voltage and collector series resistance. For this purpose, the collector connector layer (buried layer) must have a thickness which lies in the order of magnitude of the starting thickness of the active silicon layer, whereby it is not possible to increase the doping of the collector connector layer into the range of 10 e17 cm3 with acceptable expenditure, as an activation of this concentration, among other factors, leads to strong out-diffusion.
Another method for manufacturing a component on an SOI wafer is known from the publication S. B. Goody, et al, xe2x80x9cHigh Speed Bipolar on S2OIxe2x80x9d, in ESSDERC 1998. This describes the manufacture of an npn transistor which has a buried, silicided collector connector layer. For this purpose, a complete silicide layer is formed on the surface of the oxide layer of a first silicon wafer which has a complete oxide layer. An SOI wafer with a complete, buried silicide layer is subsequently manufactured by means of wafer bonding, by bonding the surface of a second silicon wafer to the silicide layer. The second silicon wafer is then thinned to the desired thickness in order to manufacture npn transistors in the remaining silicon layer.
The disadvantage of the process architecture described is that this method cannot be extended to the integration with MOS-transistors with acceptable expenditure, as a complete silicide layer substantially worsens the electrical properties by increasing the parasitic couplings, such as for example the delay time. Furthermore, it is difficult to bond the surface of the complete silicide layer to the surface of another wafer firmly and without offsets.
Furthermore, the process architecture for an npn-HBT for a silicon wafer without an insulating intermediate layer is described in the publication DE 196 09 933, in which the current amplification is increased by creating an epitactic, relatively low-doped emitter layer which suppresses the tunneling of the minorities into the emitter. Furthermore, limiting frequencies above 50 GHz are achieved at relatively low collector currents with a relatively thick, highly-doped collector connector layer (buried layer), which reduces the connection resistance of the collector, and the adaptation of the collector thickness and the doping. Furthermore, a high standard of reproducibility of the electrical parameters is achieved in the manufacturing process by means of a so-called xe2x80x9cinside-outside spacerxe2x80x9d-technique and avoiding dry etching processes on the emitter windows. The disadvantage of this is that the manufacturing process, which is exclusively oriented to the manufacture of bipolar transistors, can only be integrated with MOS-transistors at great expense. Moreover, further improvement of the HF characteristics is hindered by the use of a conventional silicon wafer as parasitic couplings occur with the substrate.
An objective of the developments in the field of semiconductor technology is to develop methods with which bipolar and HB-transistors for super high frequency applications can be combined with such FE transistors, that have extremely short delay times, without diminishing the electrical properties of any of the types of components involved. On the one hand this requires, for example, very thin active silicon layers with a thickness in the range of 0.2 xcexcm and less for the MOS-transistors, whereas on the other hand silicon layers thicker than 0.4 xcexcm are required for the bipolar transistors. Furthermore, the parasitic couplings to the substrate must be suppressed to the greatest possible extent. An important area of application for such combinations of different types of components is the manufacture of highly integrated circuits, which not only have very fast digital signal processing but also make HF outputs available at super high frequencies.
The object of the present invention is to provide a method of the type stated at the beginning with which different types of high frequency components can be manufactured together on one wafer with acceptable expenditure.
The above objects have been achieved according to the invention in a method of manufacturing components on a multi-layered silicon wafer, involving bonding together two initial wafers by a wafer bonding process.
According to this, the essence of the invention lies in bonding two wafers to form a new wafer which has buried silicided areas and an insulating intermediate layer, and components of various types, such as for example bipolar transistors as well as FE transistors with in each case optimized HF parameters, are integrated in the uppermost silicon layer on one wafer. For this purpose, a silicon wafer with an insulating intermediate layer is created on a silicon wafer, consisting of a first silicon wafer comprising a surface layer on top of an insulating intermediate layer arranged on a substrate and a second silicon wafer with a surface, by bonding the respective surfaces of the two wafers and, before bonding, at least one silicided area is created within or on the surface of the surface layer of the first silicon wafer, and subsequently at least one insulating layer is applied to the surface of at least one silicon wafer, and after bonding the substrate and at least part of the insulating layer of the first silicon wafer is removed, and then at least one component is manufactured in a series of process steps.
Investigations by the applicant have shown that it is advantageous for the manufacture of high frequency components to select a thin surface layer, preferably with a thickness less than 1.0 xcexcm. Furthermore, it is advantageous to protect the surrounding regions by means of an oxide mask during the manufacture of the silicide areas, and to create the silicided areas in different depths through further process steps, such as for example silicon etching.
An advantage of the new method in comparison to the previous state-of-the-art is that, before bonding, only those areas on the first wafer are silicided in which the insertion of a silicide layer will improve the electrical properties of the components which are created in a manufacturing process subsequent to the bonding. The silicided areas are buried by bonding the two wafers, whereby the areas lie at different depths within the surface layer. In particular, the buried layers which serve as connecting layers for components can be replaced by means of the buried silicide areas. As the conductivity of silicides is substantially greater than that of doped silicon, the thickness of the uppermost silicon layer and/or the connection resistances can be reduced. The differences in the thickness of the surface layer, which are required by the different types of components, are substantially reduced. Furthermore, the electrical and in particular the HF characteristics of vertically structured components are improved.
In a development of the method, it is advantageous to create the insulating intermediate layer, which preferably has a thickness greater than 0.2 xcexcm, by means of deposition and to apply it exclusively to the surface of the first wafer, as this makes it unnecessary to process the second wafer before bonding. Moreover, it is advantageous for certain regions of the insulating intermediate layer to have a different thickness, in particular a greater thickness in those areas in which the components manufactured in the surface layer require a particularly high decoupling from the underlying substrate material.
In another development of the method, the thickness of some areas of the surface layer is changed before or after bonding in order to adapt them to the various requirements of the electrical parameters of the individual types of components. For this purpose, starting from a very thin silicon layer, which is for example less than 0.2 xcexcm, the thickness of the surface layer is increased in certain regions by means of a selective epitaxy, or starting from a thick surface layer, which is for example in the range of 1.0 xcexcm, the thickness of the layer is reduced in certain regions by creating a thermal oxide, for example a so-called LOCOS oxidation. In conjunction with the replacement of the buried layer areas by silicides, the thickness of the surface layer can be advantageously adapted to the electrical parameters of the individual component types which are manufactured after bonding.
In a development of the method, an MOS-transistor is created as a generic component. For this purpose, it is advantageous if the surface layer is very thin and the source and drain zones of the NMOS or the PMOS completely penetrate the surface layer of the first silicon wafer (fully depleted), as such transistors have particularly short delay times and switching times. The source area can be short-circuited to the body area of the transistor by means of a buried silicided layer in order to improve the HF characteristics, such as for example the switching time of the transistor.
In another development of the method, a bipolar transistor, which preferably has a vertical structure, is manufactured as a generic component. Both PNP and npn type transistors can be created with this development. In this case, the transistors are manufactured in the regions of the surface layer which have a thick surface layer in comparison to the regions with the MOS-transistors. The collector connector of the transistors is formed by means of a buried silicided area in order to lower the saturation voltages of the transistors. Furthermore, at low voltages, the voltage drop in the transistor is reduced by the reduction of the collector series resistance so that the maximum limiting frequency is reached at low collector currents and high HF outputs can be achieved at low voltages with HF transistors, such as for example a heterobipolar transistor.
In a development of the method, a capacitor is manufactured as a generic component, in which a buried silicide area is used as the first capacitor plate in an advantageous manner. For example, layers of oxide or nitride may be used as the dielectrical layer, which may be created during the manufacturing process for other components. Moreover, a further silicide layer, as well as a metalizing layer or a doped polysilicon layer may be used as the second capacitor plate. Furthermore, it is advantageous to insert the silicide layer deep into the surface layer, for example by trench etching, before bonding in order to place the silicide layer near the surface after bonding.
In another development of the method, it is advantageous for the creation of complex integrated circuits to manufacture MOS-transistors together with bipolar transistors, in particular HB-transistors, on one wafer, and to electrically isolate the individual components from one another by means of trench etching. As the boxes so created are isolated, the components lying in the boxes may be operated at different electrical potentials. The trench etching is preferably performed by means of a trench process which has a good selectivity to the insulating intermediate layer. An inexpensive and simple trench process may be used as a complete silicide layer is not present. A further advantage is that no metallic ions are released during the trench etching, that is especially high temperature resistant silicides can be created for burying the layers, the slightest traces of the metallic ions of such silicides, such as for example cobalt or nickel, cause very damaging contamination of the components.